1. Field of the Invention
The present invention relates to a method for compressing and transmitting image raster data, whereby a data stream of image raster data is generated page-by-page from language elements of a graphics language, the data stream containing gray image areas in the form of dither cells whose gray scale values are determined by model dither cells. The invention also relates to a system for compressing and transmitting image raster data upon employment of the aforementioned method. The invention also relates to a computer system with whose assistance a computer can implement the method.
2. Description of the Related Art
It is not only black/white structures but also gray image areas that have a prescribed gray scale value that are being increasingly being employed when printing text and drawings. When, for example, an RIP module generates a data stream of image raster data page-by-page from language elements of a graphics language, for example the known printer language POSTSCRIPT, a noticeably larger dataset must be processed due to the gray image areas that are produced by dithering. The RIP module (RIP=raster image processing) is generally arranged outside the printer; this means that its image raster data must be compressed in order to be able to transmit the data in time-conforming fashion with a given data transmission rate. The following example makes this clear: A DIN A4 page contains approximately 4.3 megabytes of image raster data given a pixel density of 600 dpi (dots per inch). A high-performance printer has the capability of printing more than 400 DIN A4 pages per minute at 600 dpi. Accordingly, a data rate of more than 28 megabytes/s would have to be governed without compression.
Up to now, the image raster data were compressed with the assistance of a standardized compression method, for example the FAX G4 compression method, and were intermediately stored and/or directly transmitted to the printer in this compressed form. For example, International Patent Application No. PCT/DE95/01293, which claims the priority of German Patent Application No. 44 34 068.0, references compression methods in conjunction with printers. The PCT Application PCT/DE95/01293 is incorporated hereby by reference. When a printed page contains either no or only a few gray raster image areas, the compression time is relatively short and the efficiency of the compression is relatively high. When, however, a page contains a great proportion of gray raster image regions, then the compression time lengthens exponentially and the compression efficiency becomes low.
Picture elements in the form of gray rasters are often generated upon employment of the dithering method. In this dithering method, gray scales are generated by employing dot patterns (rasters). The dithering method exploits a unique property of the human eye: individual picture elements are no longer perceived beyond a specific viewer distance and a specific dot density, but blur to form a gray scale value. A dither cell, accordingly, contains a plurality of picture elements; of which only one picture element, several picture elements or all picture elements of a dither cell are inked dependent on the desired gray scale value. In order to achieve a good melding of the picture elements, the inked picture elements are scattered according to a predetermined algorithm. The gray scale values themselves are fixed by given model dither cells. When a dither cell contains 8×8 picture elements in the form of a matrix and a symmetrical arrangement of the inked picture elements is prescribed, then 32 or 64 gray scale values can be realized. Since dither cells and the distribution of the inked picture elements are relatively complex, standard compression methods for the reduction of the data volume often fail.
Section 6 in Das Druckerbuch of Océ Printing Systems GmbH, 3rd Edition, 1998, ISBN 3-00-001019-X describes raster technique, whereby a dither technique is also addressed. Among others, the dot pattern method is described as a dither technique. The above document is incorporated herein by reference.
The dithering method is also described in Das groβe Data Becker Computer Lexikon, 1997 Edition, ISBN 3-8158-1575-4, and in Computer Lexikon, Verlag C. H. Beck, Munich, ISBN 3-406-39696-8.
U.S. Pat. No. 5,073,953 discloses a method for the automatic segmenting of documents. The picture elements of the document are analyzed according to different types, for example black/white texts, graphics elements, continuous tone images, half-tone images, etc. The document to be analyzed is divided into sub-images and the type is assigned to these sub-images.
Image compression devices are known from German Patent Document DE-C2-38 24 717 and from the publication by W. Crocca et al., “Compression of grey digital images using grey separations”, Xerox Disclosure Journal, Vol. 15, No. 6, November/December 1990, pages 481-482. German Patent Document DE-C2-41 27 920 discloses an image processing device wherein image data are subdivided into blocks and the blocks are sequentially processed. German Patent Document DE-C2-29 53 109 and German Patent Document DE-A1-42 15 157 disclose image reception devices. Japanese Patent Document JP-A-11-65793 discloses a method with which data are compressed differently according to the object type (image or text). The contents of the above-cited documents are herewith likewise incorporated by reference into the present specification.
European Patent Document EP 0 774 858 A3 discloses a method for compressing and transmitting image raster data wherein picture elements (pixels) are combined into macro-cells in the fashion of tiles. These macro-cells are assigned to predetermined type classes, for example the type text, graphics, gray scale image, etc. The compression method to be applied is adapted dependent on the information about the type of macro-cell. For compression, the pixels of the respective tile are re-ordered (rescanning), whereby the respectively exposure value of the pixel is defined dependent on the position of the pixel with reference to the center of the macro-cell.
U.S. Pat. No. 5,465,173 is directed to an image processing method wherein half-tone image data are stored. The storing ensues block-by-block with a prescribed plurality of pixel data. A compression of the half-tone image data ensues on the basis of the block-by-block data, whereby the memory requirement is reduced.